This invention relates to a method for manufacturing sensors, with particular, but by no means exclusive, reference to gas sensors comprising conducting polymers (CPs).
Gas sensors which employ conducting polymers (CPs), such as polypyrrole and substituted derivatives thereof, are well known (see for example, International Publications WO 96/00384, WO 96/00383, and K C Persaud and P Pelosi in xe2x80x9cSensors and Sensory Systems for an Electronic Nosexe2x80x9d, pp 237-256, eds J W Gardner and P N Bartlett, 1992, Kluwer Academic Publishers, Netherlands, and references therein). Typically, a single gas sensor is produced by depositing a layer of CP between a pair of electrodes, the sensor being operated as a chemiresistor, i.e. the presence of a gas is detected by measuring variations in the dc resistance of the CP, these variations being caused by adsorption of the gas onto the CP. It is also possible to detect ac impedance properties of the CP. It is common for a plurality of sensors to be incorporated into a single gas sensing device. Each sensor has a different CP and/or a different dopant anion, and thus each sensor displays different response characteristics. The use of such arrays of CP sensors allows gases, vapours and odours to be recognised by the response xe2x80x9cfingerprintxe2x80x9dxe2x80x94the pattern of sensor responses across the array. In this manner, an impressive range of gases can be detected and identified xe2x80x94either individually or as components of mixturesxe2x80x94with good sensitivity.
One problem associated with gas sensors of this kind is that of sensor to sensor reproducibility. Typically, the polymers are produced in their conducting state by electrochemical deposition of the polymer onto and between the electrodes. However, it is very difficult to standardise the deposition conditions, and hence the precise characteristics of the deposited CP, due to extraneous variations in parameters such as deposition voltage and current, monomer concentration, and dopant ion concentration. As a result; a batch of sensors of identical type which have had the same CP electrochemically deposited thereon, but at different times, may display variations in their response characteristics and basal resistances.
Gas sensor arrays of the type discussed can be monolithic, i.e. all of the gas sensors in the array are formed on a common substrate. Thus, another problem is that if a single sensor in the array malfunctions. It is not possible to replace the single sensorxe2x80x94the entire array must be replaced.
Another problem still is that the manufacture of gas sensing arrays using electrochemical deposition is a laborious process. Each array is manufactured separately and each sensor in the array comprises a different CP/dopant ion combination. Therefore, the production of x sensor arrays having y sensors in each array requires x,y separate electrochemical deposition processes.
The present invention overcomes the above mentioned-problems.
According to the invention there is provided a method for manufacturing a batch of sensors comprising the steps of:
providing a substrate which has a plurality of conductive tracks formed thereon;
electrochemically depositing an active sensing material over the substrate and the conductive tracks in a process in which the conductive tracks are part of a single electrical circuit;
removing deposited active sensing material from predetermined portions of the substrate; and sub-dividing the substrate to produce a plurality of sensor units.
In this way, electrochemical deposition conditions are identical for all of the eventually produced sensor units. Furthermore, a large number of sensors can be conveniently and efficiently produced in a few straightforward steps.
Preferably, the conductive tracks may be held at a common potential during the electrochemical deposition. One or more conductive tracks may be formed on the substrate so as to short circuit the other conductive tracks.
The sensors may be gas sensors.
The active sensing material may be conducting organic polymer (CP). Two or more layers of CP may be deposited on the substrate. The first layer of CP may be formed by a chemical oxidative process, and may be produced by a spin coating process.
The substrate may comprise silicon or ceramic.
The removal of deposited active sensing material may be performed by etching. Plasma etching or chemical etching may be used.
The substrate may be sub-divided by sawing or scribing the substrate to produce a plurality of sensor units.
A pattern corresponding to the position of the sensor units may be cut into the substrate prior to the sub-division of the substrate. The pattern may be cut before the step of electrochemically depositing the sensing material.
The pattern may be cut by sawing or scribing.
The substrate may be sub-divided by applying pressure to the substrate.
Heating elements for the sensor units may be incorporated into the substrate prior to the electrochemical deposition.
The batch may comprise at least one hundred sensor units, preferably at least three hundred, most preferably at least five hundred sensor units.
Methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: